2. SWITCHING GLASSES WITH NANO PARTICLES
Introduction
• The surface layers of certain silicate glasses containing 𝐵𝑖2𝑂3have been reported to show “memory
switching” after they were subjected to sodium silver ion-exchange, which was followed by a reduction
treatment in hydrogen .
• Such glasses before ion-exchange and reduction stages have a microstructure consisting of metallic bismuth
particles with diameters in the nano range : 5 nm to 25 nm dispersed in an amorphous matrix.
• After ion-exchange and reduction treatments, the nano particles of silver of maximum diameter 100 nm are
also precipitated.
• The measurements carried out on ‘thick films’ of these glasses, which have not been subjected to any ion-
exchange and reduction treatments, show that the switching action is the characteristic feature of the base
glass itself.
• Here, the role of nano silver particles being only to lower the switching voltage .
• This is some sort of an achievement, since we want a flow of current always at a lower voltage to make the
‘switching device’ efficient.
3. Preparation of Glasses with Nano Particles
• For bismuth-containing glasses, most of the work was done on a composition : 10 Na2O, 18 B2O3, 64 SiO2,
8 Bi2O3 (mole%). These glasses were identified as showing ‘memory switching’ .
• Some work was also carried out on samples of composition : 25 Na2O, 10 CaO, 55 SiO2, 10 Bi2O3 (mole%)
to show that the electrical behaviour due to nano metallic bismuth is independent of the chemical composition
of the glass matrix.
• Finally, in order to show the effect of nano-bismuth particles more clearly, some work was also done on the
‘base glass’ compositions : 10 Na2O, 26 B2O3, 64 SiO2 and 25 Na2O, 20 CaO, 55 SiO2 (mole%)
respectively.
• The compositions of different glasses, which were used to study the effect of nano-bismuth particles on their
electrical properties are summarized in Table 6.1, and which are numbered as 1 to 4.
• The compositions of the glasses, which were used to elucidate the effect of the dispersion of nano-crystalline
selenium particles on the resulting composite matrix, are summarized in Table 6.2, and which are numbered
as 5 to 8.
4.
5. • The glasses were prepared by melting reagent-grade chemicals in alumina crucibles in an electrically heated
furnace at temperatures ranging from 1200 to 1400°C under normal atmospheric conditions.
• The selenium purity was 99.9%, which was introduced as powder in the mixture. The glasses were quenched
by pouring the melts onto the aluminium moulds. These were then annealed at 500°C for an hour and then
cooled slowly to room temperature within the furnace after being switched off.
• A substantial amount of selenium was lost during melting, because of the low melting temperature of these
selenium containing glasses (i.e. nos. 5 to 8) . Therefore, the final compositions of these samples were
determined after estimating their selenium contents by standard chemical methods.
• For preparing glass 8, which contained the largest amount of selenium, the oxide glass was first made with
adequate molar percentages of different components.
• This glass was then powdered and mixed with the appropriate amount of selenium powder in order to get the
required composition.
• The mixture was taken in a quartz tube, which was evacuated and then sealed.
• The sealed quartz ampoule was heated to a temperature to 1000°C for 24 hours with gentle rocking for
homogenization purposes.
• The quartz ampoule was taken out and allowed to cool at room temperature. The optical absorption spectra of
the glasses 5 to 8 confirmed that selenium was present in its elemental form and not as an oxide .
6. • For electrical measurements, circular metallic electrodes (both aluminium or gold giving the same results) of
diameters varying between 1 and 2 cm were evaporated onto two faces of the samples, which were ground
and polished to a thikness in the range 1 to 2 mm. Fo glasses 5 to 8, silver paint was used as electrodes. The
DC resistivity was measured by plotting their voltage-current characteristics over a decade of voltage by using
an ‘Electrometer’ (Type 1230A, General Radio, U. K.). The linearity of the I-V curves was obtained for all the
samples in the temperature range – 160°C to +200°C.
• For AC resistivity measurements, a guard-ring was also deposited on one of the sample faces, and three-
terminal measurements were carried out over a frequency range 100 Hz – 100 KHz with a ‘Capacitor Bridge’
(Type 716C General Radio, U.K.). For measurements at higher frequency, i.e. between 100 KHz and 30 MHz
, a ‘Boonton Q-Meter’ (Type 260A) was used.
• The possibilities of switching phenomena in selenium containing glasses were explored by subjecting them to
a Na+Ag+ ion exchange, which was followed by a reduction treatment in hydrogen. The electrical
measurements on these samples were carried out by the method described elsewhere.The I-V characteristics
of these samples were carried out by a ‘Transistor Curve Tracer’ (Type 575,Tektronix, USA). For all the
electrical measurements on selenium containing glasses, the samples were kept in a dark chamber so that
‘photo-conductive’ effects could be avoided.
7. Electrical Data of Nano Particles of Bismuth and Selenium
Electrical Conduction in Bismuth Glasses
• The plot of log vs. 1/T is shown in Figure 6.1 for the glasses 1 to 4.
• It is seen that for glasses 1 and 3 containing nano particles of bismuth, there are two linear curves, whereas for the glasses 2
and 4 containing no bismuth, there are simple linear plots over the entire temperature range. These results indicate that for
the glasses 1 and 3, there are two conduction mechanisms operative in two different temperature ranges.
• By assuming an Arrhenius type of variation of resistivity as a function of temperature,which is written as :
8. • The activation energies of these
glasses are estimated in the two
different temperation ranges, i.e.
between 20° - 200°C and 200° -
500°C, which are shows in Table -
6.3.
• It is evident from the table that the
resistivity of the glasses 1 and 3 in
the higher temperature range arises
due to the movement of Na+ ions
through the glass matrix, which
obviously requires a higher
activation energy due to the
'diffusional' jump of Na+ ions in
the glass matrix [6].
9. • In oxide glasses, it is known that if the ratio of the network former (i.e. SiO2, B2O3, etc.) to network modifier (i.e. Na2O, CaO,
Bi2O3, etc.) is reduced, the concentration of non-bonding oxygen ions is increased.
• This loosens the structure that makes it congenial for Na+ ion jump, which involves lower activation energy. The slightly lower
activation energies of the glasses 1 and 3 compared to those of glasses 2 and 4 can be attributed to the fact that the ‘coherence of
silica network’ in the former is reduced owing to the presence of Bi2O3 in these glasses, which act as network modifier . There are
various cases possible for the ‘electrical conduction’ due to Na+ ions in the higher temperature range, which can be discussed as
follows :
• 1. Between Glasses 1 and 2
Both the contents of silica and soda are constant, but borax is increasing from 18 to 26 mole%, which stiffens the network structure
(total network former = 64 + 18 = 82% in glass 1 and 90% in glass2) making the Na+ ion jump difficult and thereby the activation
energy increases from 1.10 to 1.42 ev (see Table 6.3)
• 2. Between Glasses 1 and 3
The total network former is reduced from 82% to 55%, and thereby the activation energy decreases from 1.10 ev to 0.90 ev due to less
structural cohesion. Moreover, the bisbuth content is higher in glass 3 than that of glass 1, which explains the effect of bismuth in
loosening the structure from the point of view of energy.
• 3. Between Glasses 1 and 4
They have the same activation energy, even if the ratio of network former/modifier is higher in the former, where the presence of
bismuth weakens the structure more. So, the effect of bismuth is more prominently seen in this case.
• 4. Between Glasses 2 and 3
• In this case, the difference of activation energy is the highest (from 1.42 ev to 0.90 ev), since the network former is substantially
reduced as well as there is bismuth present in glass 3, which makes the ‘weakening’ of the structure maximum and hence the lowest
activation energy.
• 5. Between Glasses 2 and 4
Here, the reduction in activation energy is from 1.42 ev to 1.10 ev due to the substantial reduction of network former from glass 2 to 4,
but the absence of bismuth in glass 4 does not allow a reduction of activation energy further. So, the effect is clear.
10. • Therefore, it is seen that the above glasses 1 to 4 containing bismuth are quite interesting for a detailed
analysis for activation energy. In the lower temperature region, the conduction in glasses 1 and 3 is via
‘electron hopping’ between isolated islands of nano-metallic bismuth particles, and hence the activation
energy is extremely low at 0.03 ev.
• Both the DC and AC resistivity data obtained at different
frequencies for glass 1 are shown in Figure 6.2.
A change in slope for the DC resistivity curve is
also observed at a temperature of 120°C.
11. Electrical
Conduction in
Selenium Glasses
• The DC resistivity data for the
glasses 5 to 8 containing nano-
crystalline selenium grains are shown
in Figure 6.3, which shows a sharp
decrease of the resistivity as a
function of selenium concentration at
low temperature.
• For these selenium containing
glasses, it is seen that the slope of the
resistivity curve changes in the
temperature range – 60°C to 120°C,
depending on the composition. The
temperature at which, this break of
slope occurs increases with the
increasing selenium content.
12. Tunneling Conduction in Nano Particles
• The present materials are very similar to ‘granular metals’, which represent a physical system for studying
‘percolation conductivity’ . The microstructural features of these “glass-conducting nano ganules” system
clearly show that the electrical transport due to tunneling mechanism between the isolated conduting particles
will influence their DC conductivity behaviour. For low electric fields, Abeles et al [9] have shown that the
resistivity of granular metals, when the particles are isolated from each other, is given by :
• where, m denotes the electron mass, the effective barrier height and h is the Planck’s constant, S is the
separation between the grains, and 𝐸𝑐
0 is the energy required to generate a pair of fully dissociated positively
and negatively charged grains. This energy is given by :
13. ELECTRONIC CONDUCTION WITH NANO PARTICLES
• Preparation of Nano Particles and Conductivity Measurements
• The three-terminal electrical measurements were made on disc-shaped samples of 1 cm in diameter and 2 to 3 mm
in thickness, with gold electrodes evaporated on to the flat surfaces, which was made by polishing through a normal
procedure.
• The DC conductivity measurements were made between 373 and 1000°K, by using a standard ‘Electrometer’.
• The AC conductivity measurements were made only at room temperature as a function of frequency between 10
KHz and 200 MHz in a Q-meter with variable frequency range.
• The conductivity measurements were also made down to liquid nitrogen temperature of 77°K for the samples heat-
treated at 700°C and 800°C respectively.
• The DC conductivity measurement was started from the low temperature end at 77°K, by first cooling the sample
with an applied electric field, and then taking the readings as the temperature was raised at a constant rates of
0.35°K/min and 1°K/min respectively for 700°C sample.
• For the 800°C sample, the rates of heating were 1°K/min and 2°K/min respectively. This experiment is known as
‘Thermally Stimulated Polarization Current’ (TSPC).
• In the ‘Thermally Stimulated Depolarization Current’ (TSDC) measurements, after cooling the sample and doing the
TSPC measurements, the sample is again cooled without any applied electric field and the readings are taken on
reheating at the rate 1°K.
• These TSDC data were only recorded for 800°C sample.
14. Impurity States in Electronic Conduction
• The dopants are invariably added as a ‘crucial element’ in all the semiconductor
devices for their proper functioning.
• The bound states are introduced in the forbidden gap due to the presence of such
dopants in a semiconductor material.
• Hence, it influences both its optical and electronic transport properties.
• Therefore, we have to properly understand the ‘impurity states’ in the
semiconductors nano-structures, which has attracted the attention of many
workers in this new field, since the impurity states are strongly dependent on the
following :
1. Nature of the impurity states,
2. Strength of the impurity states, and
3. Width of the confining potential,
15. Optical Properties
Introduction
From the ancient times, the glasses are known as important and useful optical materials.
The glasses show all types of usual optical properties, with which we are generally concerned in our daily life,
e.g. refraction, reflection, transmission and absorption.
However, there are many crystalline materials with and without centro-symmetric properties, which also show
erxcellent optical properties due to their dielectric properties, i.e. the existence of the ‘electric charges’ and their
spatial ‘displacements’.
While this so-called displacement gives rise to polarization, which in turn affects many important properties,
that which will be discussed subsequently, like electro-optic and acousto-optic properties.
OPTICAL PROPERTIES
Some Definitions
The propagation of an electromagnetic wave in a material produces a displacement of electrical
charge. For a sinusoidal wave, the change of speed and intensity are continuous in the complex
refractive index (n*), which is related to the complex permittivity :
ε* = ε′ + iε′′
16. It is given by the relation as : 𝑛∗2
= ε*
By writing, n* = n + ik,
where n is the refractive index and k the absorption index ,
we get : 𝑛2 – 𝑘2 = ε′ and,
2nk = ε′′
the quantity (ε′ – 1) = χ is the electrical susceptibility.
The Refractive Index and Dispersion
The refractive index is equal to the ratio of the speed of incident light in the vacuum (𝑣0),
with respect to that in the material (𝑣𝑚), n = 𝑣0 / 𝑣𝑚. It depends on the wavelength and
normally, it decreases as the wavelength λ increases. This variation carries the name of
‘Dispersion’ and can be defined by the relation :
D = dn/dλ
The variation of n and k are related to each other. The refractive index actually changes in an inverse
sense compared to the dispersion in the region of strong absorption, which is called “Dispersive
Anomaly”.
17. The Non-Linear Refractive Index
The dependence of the refractive index on the intensity of light is governed by the electrical
susceptibility of the third order. The refractive index is related to the average electric field
of 𝐸2as follows :
n=𝑛0 + 𝑛2 𝐸2
where 𝑛0 is the usual linear index and
𝑛2 is the coefficient of non-linear refractive index.
The importance of non-linear index in the technology of making “High Power Lasers” has
driven the scientists to search for glasses possessing a smaller value of 𝑛2 .
It has been possible to show that the glasses with a smaller index and with a weakest
dispersion behaviour like the 'fluoroberyllates' possess the smallest value of 𝑛2 .
20. SPECIAL PROPERTIES
Accidental Anisotropy-Birefringence-Elasto-Optic Effect
• In certain specific conditions, the glass can become anisotropic.
• The most frequent reason is the application of a mechanical stress, which induces a
birefringence.
• The speed of propagation, i.e. the refractive index, thus depends on the orientation of the
plane of polarization.
• Under the action of an axial stress (𝑧), the glass behaves like a uniaxial medium.
• It is diagrammatically shown in Figure 7.1. The speed of propagation of light parallel to
𝑧 is identical whatever be the orientation of the plane of polarization., whereas for a ray
of light perpendicular to 𝑧, the speed varies depending on whether the plane of
polarization is perpendicular to 𝑧 (Ordinary Ray = OR) or parallel to 𝑧 (Extraordinary
Ray = ER).
21.
22.
23. Electro-Optic and Acousto-Optic Effects
There are systems, which are based on Laser technology that will need a set of extra
technical items to the Lasers and wave guides.
We can cite examples of devices that modulate, deflect, switch, translate in
frequency, and also modify ‘optical signals’ in a manner, which can be controlled
and predicted.
The requirements in this field have resulted in the development of materials that are
capable of ‘optical communicaton’ with a very low loss in transmission.
These types of optical properties of a material can be ‘changed’ by various fields
interacting with the ‘optical signal’, e.g. by electric field (called electro-optic) or by
magnetic field (called magneto-optic), or even by an externally applied stress, i.e.
elasto-optic, which is already discussed above, i.e. birefringence.
24. The Electro-Optic Effect
When an electric field, which may be ‘static’, ‘microwave’ or even an ‘optical
electro-magnetic’ field, interacts with the ‘optical signal’ to produce a ‘change’ in
the “Optical Dielectric Properties”, then an “electro-optic” effect occurs in the
materials.
In certain crystalline materials, the electro-optic phenomenon arises due to
‘electronic’ effect and in some other materials, it is mainly due to the ‘phonons’, i.e.
the vibrational modes of the atomic system.
This kind of effect in certain cases may be due to a variation in linear fashion or in a
quadratic manner with the electric field.
25.
26. So, the above description is for a linear case.
However, for a crystal system lacking a ‘centre of symmetry’, e.g. a ferroelectric crystal like
lithium niobate and lithium tantalate, the electric field can be also expressed in non-linear terms
by involving the ‘polartizability’ of the atomic system , since in this case, the polarization is not a
linear function of the applied electric field.
Hence, it is expressed up to the third order as :
27. (a) Crystal Symmetry,
(b) Direction of the Applied Electric Field, and also on
(c) Propagation and Polarization Direction of the Optical Beam.
As mentioned earlier, there are various important electro-optical materials like lithium
niobate, lithium tantalate, potassium tantalate-niobate, calcium niobate, strontium-barium
niobate, barium-sodium niobate, etc. In many of these crystals, Nb or Ta ion is
octahedrally coordinated with six oxygen ions, which form the basic structural unit.
The main property of the 'change' in refractive index with an applied electric field is
exploited in the electro-optic materials in terms of a variety of applications such as :
(a) Optical Oscillators,
(b) Frequency Doublers,
(c) Voltage-Controlled Switches in Laser Cavities, and of course
(d) Modulators for Optical Communication Systems.
28. The Acousto-Optic Effect
In the above example of electro-optic materials, the refractive index changes with an applied
electric field. Instead of electric field, if a crystal is strained, then also the refractive index can
effect a change. This change of refractive index by strain is known as ‘acousto-optic’ effect.
The crystal lattice has a potential, which can be changed by the action of strain that changes the
shape and size of the molecular orbitals of the weakly-bound electrons. This causes a change in the
polarizability and refractive index as well.
In a polarizable crystal, the strains have different values at different spatial directions, which are
ultimately expressed as a strain tensor. Hence, the effect of the strain on the indices of refraction of
a crystalline lattice depends on the direction of these ‘strain axes’ and also on the direction of the
‘optical polarization’. These spatial dependence eventually guide the acousto-optic properties of the
nano crystalline materials.
29. If a plane elastic wave is excited with a given crystal system, a periodic strain effect occurs with a
spatial extent that is equal to the acoustic wavelength.
Then, due to this strain effect, an acousto-optic variation of the refractive index occurs in the
crystalline lattice, which is equivalent to a ‘volumetric diffraction grating’.
Based on this principle of partial diffraction of light incident on an aousto-optic grating at a
proper angle, the acousto-optic devices are made.
In an acousto-optic device, the use of a particular crystal depends on many factors, such as :
(a) The ‘piezoelectric’ coupling (produced by the strain in the crystal),
(b) The ‘ultrasonic’ attenuation, and also on
(c) The ‘acousto-optical’ coefficients.
The important acousto-optic crystals are lithium tantalate, lithium niobate and some other
leadbased compounds. The refractive index of these material is about 2.2, and they are also
transparent in the visible spectrum, i.e. 400 nm to 700 nm of wavelength. There are single domain
and multi-domain materials, which have been superbly crafted by means of ‘domain engineering’
in the nano scale, which makes an ‘integrated optical device’ very efficient with precise control.
30. THE COLOURED GLASSES
• If a glass “selectively absorbs or scatters light” in a part of the visible spectra
(VIBGYOR), it results in an irregular transmission of light and the glass appears to
be coloured to the human eyes, which is sensitive only in the wavelength range of
400 nm to 700 nm.
• The impression of colour is actually a subjective “sensation” depending on the
‘spectral sensitivity’ of the human eye on the one hand and on the nature of the
incident light on the other hand.
• By contrast, a spectral transmission curve is perfectly defined and is physically
measurable. It can serve as a quantitative indication on the colouration of the glasse
31. Absorption in Glasses
In the common glasses, the absorption in the visible is primarily due to the transition metal (TM)
ions, which are characterized by their incomplete 3d shells, particularly, V, Cr, Mn, Fe, Co, Ni,
Cu, etc. and to a lesser degree due to the presence of rare earth (RE) ions with incomplete 4f
shells, and in some cases due to colour centres.
This is guided by the ‘Ligand Field Theory’ in which it is postulated that the degeneracy of the
‘electronic levels’ will be lifted by the electrical field of the anions (i.e. oxygen ligands in the
oxide glasses) surrounding the transition metal cations.
The colours produced depends on the oxidation state or redox state, and on the coordination
number of the concerned ions. For example, 𝐶𝑜2+
in a silicate glass is in a tetrahedral
coordination with oxygen ions and produces a deep blue colour, whereas in a metaphosphate or
borosilicate glass with coordination number of 6, it gives rise to rose colour.
The rare earth ions like Y, La, Gd, Yb and Lu give rise to colours due to the separation of levels in
their 4f shells. These ions do not have any band in the visible region of the spectra. By contrast,
Nd gives a strong red-violet colouration. The glasses doped with 𝑁𝑑3+ ions are the basis of
“Laser Glasses”. In the same way, the Pr ion gives rise to a green colour and the Er ion gives rise
to a rose colour.
32. The rare earth ions like Y, La, Gd, Yb and Lu give rise to colours due to
the separation of levels in their 4f shells.
These ions do not have any band in the visible region of the spectra. By
contrast, Nd gives a strong red-violet colouration.
The glasses doped with 𝑁𝑑3+
ions are the basis of “Laser Glasses”.
In the same way, the Pr ion gives rise to a green colour and the Er ion
gives rise to a rose colour.
33. The Colour Centres : Photochromy
A prolonged exposure of a glass to the UV radiation of the Sun produces
a colouration due to the change of valence of certain ions or
combination of ions. This is called the phenomenon of “solarisation”. If
the glass contains Mn and Fe as impurities, we can write :
where hν is the photon energy of UV and 𝑒−
is the ejected electron,
which is arrested somewhere in the glass structure, for example, on a
site relative to 𝐹𝑒3+
as :
34. The “solarised colour centre” thus becomes stabilized and the glass takes a taint of violet
colour, due to the presence of 𝑀𝑛3+.
This has been observed in the ancient glasses, which have been subjected to a prolonged
exposure of light.
The highly reduced silicate glasses containing 𝐸𝑢2+ and 𝑇𝑖4+ under the action of photon
energy develop the colour centres, which progressively disappear as the source of light is
cut off.
These glasses are called “photochromic” glasses. In this case, the reaction is as follows :
and the colour centres responsible for giving colour is due to 𝑇𝑖3+ ions. The other example
of ‘photochromism’ refers to the presence of nano-crystalline particles forming a well
dispersed phase in the glass, which is ultimately responsible for colouration.
The colour can also result from the absorption of light by the interaction with the electrons,
which are not associated with any specific ions, but arrested by the ‘structural networking
defect’.
A typical example is ‘colour centre’ in many crystals. For glasses, the variety of sites to
arrest them inside the glass makes the transition generally enough uniform across the
spectra, which produces an uniform darkening (grey tinge) rather than a well defined colour.
35. The bombardment of a glass by the energetic particles or by irradiation
by the X-rays or γ-rays produce a modification of the transmission,
which can be suppressed by a thermal treatment of annealing, which is
normally called thermal whitening or leaching.
The atomic mobility thus becomes sufficient for the restoration of an
‘unperturbed structure’.
This progressive modification of the transmission under irradiation can
be troublesome for certain technical applications, i.e. nuclear reactors,
space technology, etc., and we look for avoiding such consequences.
In other cases by contrast, one looks for deliberately producing it, for
example, in the case of variable transmission for lenses for spectacles
and the glass for radiation dosimeter.
36. The Colour due to the Dispersed Particles
The glass consists of a medium where it is possible to produce a variable
precipitation reaction under the influence of heat treatment or action of light,
i.e. the photo-sensitive reaction. The ions of certain metals like Cu, Au, Ag, Pt
dissolving in the glass can be reduced to a metallic state by incorporating
reducing agents like tin oxide or antimony oxide in the composition.
The Gold Ruby Glass
𝐴𝑢3+
+ 3𝑒−
→ 𝐴𝑢0
and the necessary electrons are supplied by the reactions as :
37. In course of this heat treatment, first of all, it produces an agglomeration of Au
atoms in the form of a “colloidal mass” and then in the form of small crystals, which
are in the nano range.
The glass, initially colourless, then takes a tinge of lively “ruby” during the final
stage of this heat treatment, which is called the “striking” treatment.
The same phenomenon occurs from the interaction of light with the metallic
particles : It does not refer to a phenomenon of “scattering”, but an absorption by the
gold sol.
The Silver and Copper Rubies
Similar absorption is produced in the glass containing silver in solid solution. The
dissolving Ag+ ions that was originally colourless can be reduced to a metallic state
Ag0 , the atoms are then fluoroscent.
The agglomeration of the Ag atoms to a colloidal state makes the fluorescence
disappear, but it provokes a yellow colouration in its place, which can again be
explained by the theory of Mie.
The corresponding absorption around 396 nm has been used in the study of
diffusion of hydrogen in the glasses doped with silver, which serves as the “tracer”.
38. The reduction of the ions like Cu+, Ag+, Au3+ can be effected by the photo-
sensitive reaction by adding a small quantity (0.05%) of photo-reducing agents like
CeO2 in the glass. Under the action of UV irradiation at room temperature, there is
an emission of an electron as :
The atoms of Cu serve as the nucleation centres and a heat treatment permits the
development of the colour in the irradiated part. Such glasses, called “photo
sensitive” glasses, containing Cu, Ag or Au are usually produced for commercial
applications. It allows the execution of real photography by using the sensitivity
exposure to the UV light, which is followed by a development of heat treatment at a
temperature of 500 - 600°C.
39. It is possible to precipitate small ‘silver halide’ crystals in a suitable glass and
to obtain a transparent glass enjoying the property of ‘photochromism’. The
typical glasses are boro-alumino-silicates containing AgCl, AgBr or AgI in
the form of small crystals of 8 nm to 15 nm, which is precipitated by a heat
treatment between 400 and 800°C. The space between the particles is of the
order of 100 nm.
The sensitivity and the kinetics of the darkening process of such glasses and
their return to normalcy are influenced by the following :
(a) The Glass Composition ,
(b) The Nature of the Halogen Ions,
(c) The Particle Size (preferably in the nano range), and
(d) The Heat Treatment Schedule.
It is known that the addition of Cu increases the sensitivity to the light.
40. The system function as a reversible “photographic plate” : the absorption of a photon
provokes a dissociation to Ag0 and halogen. The metallic Ag0 absorbs the light and colours
the glass in grey. Contrary to the usual photo-sensitive layer in photography, the pair can
recombine as the light is removed, which produces a “whitening” or “Colour Leaching”.
Such systems are perfectly reversible and do not show the sign of “fatigue” up to about
300,000 cycles of ‘darkening-whitening’ cycles, which is contrary to the organic
photochromic substances, which are found to be progressively degraded (i.e. ageing). These
glasses find important applications in the glass lenses for spectacles.
The Luminescent Glasses
The colour of glasses just studied show the phenomenon of absorption in the visible spectra.
The colours due to the “fluoroscence” is shown by the electronic transition with an emission
of a photon in the visible. One atom brought to an excited state by the absorption of a
photon returns to its fundamental level with the emission of light → either immediately, i.e.
fluoroscence, or after a significant delay, i.e. phosphoroscence.
41. The centres of fluoroscence in the glasses can be either the metallic atoms in the
“nano range” (e.g. Ag atom), or the crystalline phases (CdS), or certain ions → the
most important of which are rare earth ions, which intervene in the amplification of
coherent (stimulated) light, i.e. “The Lasers”.
The metal nano particles stabilized by organic molecules are now creating a new
class of materials that are different from both conventional bulk materials and the
atoms, giving one of the smallest building blocks of matter. The stabilizers play
important roles in not only protecting the metal nano-particles, but also controlling
the properties for optical functions.
The ‘stimuli-responsive’ colour change of colloidal dispersions of Au nano particles
as an application as sensors : The gold nano particles protected by 3-
mercaptopropionic acid were prepared by reducing tetrachloroauric acid in the
presence of 3-mercaptopropionic acid. The colour of the dispersions changed from
red to purple by adding hydrochloric acid, and changed back from purple to red with
the addition of an aqueous sodium hydroxide solution. The change responsive to pH
is reversible even after 5 repetitions. On the other hand, the colour of colloidal
dispersion of Au nano particles stabilized by poly(beta-cyclodextrin) (PCyD)
changed from red to purple with the addition of mercaptocarboxylic acid, suggesting
the inclusion complex formation of the PCyD protecting Au nano particles with
mercaptocarboxylic acid.
42. The Laser Glasses
A solid Laser is a ‘luminescent material’ wherein the light emitted by the
fluoroscence from one of the “centres” stimulates the other ‘centres’ on its own in
order to provoke the emission of light in phase with that of the first “centre” and in
the same direction.
In order to obtain such a stimulated emission, it is necessary to provoke an
“inversion of population”, i.e. to create a situation wherein the species in the excited
state are more in numbers than that in the fundamental or lowest energy state, so that
the ‘population inversion’ can take place.
By limiting to the case of only excitation by photon, which is called optical
pumping, it can be shown that it is necessary that the “excitable” ions dispose at
least “three” energy levels, as shown in Figure 7.2.
43.
44. This excitation, i.e. the optical pumping, elevates the atoms to the level 3 (or 3′),
wherein they have a chance to return either to the fundamental level with emission of
a photon, or to move to an intermediate level 2, by a non-radiative transition.
This level 2 of fluoroscence is of fundamental importance to understand the
mechanism of Laser. As the atoms return from the level 2 to the base level 1, it emits
a light of the same wavelength as the ‘atom’ which has originally stimulated this
transition, and this ‘atom’ in turn stimulates another transition and so forth.
Hence, the process continues. In the absence of the level 2 one could only have an
equalisation of population density between the levels 3 and 1. Some systems have
four levels.
The effect of Laser is produced between the levels 2 and a level 1 above the
fundamental level 0. The excitation is generally produced by an ‘external lamp’,
which emits a light, which is absorbed by the ‘excitable’ ions.
45. In actual situation, the active solid is placed between two ‘mirrors’ with the reflectivities as : 𝑅1= 100%
and 𝑅2 < 100%. The light emitted between the mirrors provokes the ‘stimulated emission’ by the
'avalenche' effect. If 𝑁2 and 𝑁1 are the ‘populations’ in the higher and lower states respectively for an
unit volume, for an inversion of population with N = 𝑁2 – 𝑁1 > 0 and a coefficient of gain/ion (β), it
can be shown that the ‘amplification’ of light will be produced if the following condition is met as :
R1 R2 . exp ((βN – α)/2L) > 1
where,
α is the normal absorption coefficient and
L the length of the sample bar.
The value of β depends on the indices of refraction n, which depends on the following :
(a) The wavelength (λ),
(b) The variation of wavelength (Δλ) of the fluoroscent rays, and also on
(c) The Einstein Coefficient (A).
46. The dependence of the Einstein coefficient (A) is expressed as follows :
β = (1/8πc) . (4
/ 𝑛2
) . (A/Δλ)
Originally, the solid Lasers were essentially the ‘rubies’ (i.e. alumina doped with 𝐶𝑟3+ +
ions), or the YAG (Yttrium Alumina Garnet) containing 𝑁𝑑3+ ions. The ‘ruby’ Laser emits
around 690 nm and the YAG around 1060 nm.
47. Some Examples of Nano Particles
Nano Particles of Tin Dioxide
The emission intensity of the peak at 612 nm of the 𝐸𝑢3+ ions due to 5 𝐷0 → 7 F2
transition, activated by 𝑆𝑛𝑂2 nano-crystals were found to be sensitive to the nano
environment for both doped and coated samples by Saha Chowdhury and Patra [14].
The luminescent efficiencies of nano-crystals of SnO2 – ‘doped’ by Eu2O were
compared with those of SnO2 - ‘coated’ by Eu2O, and the intensities were found to
be significantly higher for coated materials. It was also found from the measured
luminescent intensity that Eu3+ ions occupy ‘low symmetry’ sites in the nano-
crystals of SnO2 – coated by Eu2O, with radiative relaxation rate much higher in the
coated sample than in doped sample due to the asymmetry of Eu3+ ions. In this
work, it is not made clear whether this sort of ‘asymmetry’ originated from the
nature of the nano-particles [14].
48. Nano Particles of Cuprous Oxide
The cuprous oxide (Cu2O) particles coated with poly-acrylamide having
diameters in the range from 4.8 nm to 8.6 nm were prepared by a
chemical method. The optical absorption of these particles was found to
be characterized by excitons with corresponding energies varying from
2.6 to 2.25 eV in the above particle size range. A second optical-
absorption maximum signifying surface states within the band gap is
exhibited by the specimens with relevant energies varying from 2.6 to
1.77 eV in this particle size range [15].
49. Nanoribbons of Zinc Sulphide
One-dimensional nano-structured materials set the pace of the recent trend in nano-
materials research due to their wide range of potential applications in many nano-
scale devices. It is known that ZnS is an important semiconductor used as ‘phosphor
materials’. Hence, some efforts have been made by Kar et al [16] into controlling
the size, morphology and crystallinity of the ZnS crystals in order to fine-tune their
physical properties. The ZnS nano-ribbons were synthesized on a Si substrate by
vapor liquid solid (VLS) process at 1100°C using gold as the catalyst, and the
synthesized products were characterized by XRD, EDAX, SEM, TEM and photo-
luminescence (PL) measurements. The ZnS nano-ribbons were found to be
crystallized with the hexagonal wurtzite phase. The nano-ribbons were ultra long
with width varying within 300-500 nm. The nano-ribbons were also found to have
good 'emissive property' with the blue emission centered at ~ 399 nm [16]
50. Nano Particles of CdS
The optical and microstructural properties of nano-composite thin films based on the
systems CdS-ZnO and CdS-Al2O3 were studied by Ray et al [17]. These nano
materials were synthesized by solgel technique. The molar ratio of CdS with ZnO
and Al2O3 were varied within the range of 20:80 to 50:50. The nanoparticles of CdS
that are highly confined with radius 1.8 nm to 4.7 nm, which were estimated from
the blue shift of the absorption edge, were obtained by using ZnO matrix, whereas
for Al2O3 matrix the size varied from 2.8 nm to 7.0 nm. The microstructural
characterization by highresolution TEM (i.e. HRTEM) revealed well-resolved
crystalline nano-particles in both cases.
